The invention relates to a data processing system comprising a central processing unit, a first memory coupled to the central processing unit for storing variable data values, an eventing engine coupled to the central processing, a debug module coupled to the central processing unit and to the eventing engine for receiving the variable data values by way of the eventing engine in real time and configured to trigger breakpoints; and an interface to connect to an external device. Both event-based variable watching and debugging can occur contemporaneously by a user using an external device connected to the interface.